Make Concurrent::Set thread-safe on CRuby

[flavorjones]

See #900 for original issue pointing out that Concurrent::Set#each (and other methods as well) are not thread-safe because the implementation was short-circuited to be ::Set.

This PR adds two commits:

• one introducing test coverage for thread-safe usage of #each combined with #add and #delete
• a second introducing a CRuby-specific Set implementation that uses Monitor to synchronize across all instance methods

This approach, using a new method ThreadSafe::Util.make_synchronized_on_cruby, borrows heavily from the approach used for Rubinius (see ThreadSafe::Util.make_synchronized_on_rbx).

Please let me know if this approach is too naïve, I fear I may be missing something obvious.

[flavorjones]

The test failures don't seem to be related to this change.

[chrisseaton]

Yes sorry it looks like the CI has bit-rotted out from underneath us. We should switch to GitHub Actions quickly.

[pitr-ch]

Replaced by #911